Process for the preparation of low molecular weight 4,4'-diphenylmethane-uretdione-diisocyanates

Abstract

Low molecular weight 4,4'-diphenylmethane-uretdione-diisocyanates corresponding to the formula: ##STR1## wherein n is a number from 0 to 1, are made by dimerizing diphenylmethane-4,4'-diisocyanate at a temperature of 50° C. or less in a mixture of non-polar and moderately polar solvents in the presence of a dimerization catalyst. The dimerization product may then be worked up at low temperatures (i.e., ≦50° C.). The dimerization products of diphenylmethane-diisocyanate thus obtained generally have only a small amount of oligomeric uretdione-diisocyanates present. These dimerization products may be used advantageously in the synthesis of polyurethanes.

What is claimed is: 
     
       1. A process for the production of a low molecular weight 4,4'-diphenylmethane-uretdione-diisocyanate corresponding to the formula: ##STR8## in which n represents a number from 0 to 1, comprising dimerizing a 4,4'-diphenylmethane-diisocyanate at a temperature less than 50° C. in the presence of a catalyst and in an isocyanate-inert solvent-mixture of non-polar and moderately polar solvents. 
     
     
       2. The process of claim 1 wherein the solubility of the 4,4'-diphenylmethane-diisocyanate in the non-polar solvent is from 5 to 25 wt. %. 
     
     
       3. The process of claim 1 wherein n represents a number from 0 to 0.5. 
     
     
       4. The process of claim 1 wherein the non-polar and moderately polar solvents are used in quantities such that the ratio of non-polar to moderately polar solvent is from 1:2 to 2:1. 
     
     
       5. The process of claim 4 wherein the non-polar solvent is an aliphatic and/or cycloaliphatic hydrocarbon and the moderately polar solvent is selected from the group consisting of aromatic hydrocarbons, chlorinated derivatives of aromatic hydrocarbons, carboxylic acid esters, ketones, ethers and mixtures thereof. 
     
     
       6. The process of claim 1 wherein the non-polar solvent is an aliphatic and/or cycloaliphatic hydrocarbon. 
     
     
       7. The process of claim 1 wherein the moderately polar solvent is a compound selected from the group consisting of aromatic hydrocarbons, chlorinated derivatives of aromatic hydrocarbons, carboxylic acid esters, ketones, ethers and mixtures thereof. 
     
     
       8. The process of claim 1 wherein the solubility of the 4,4'-diphenylmethane-diisocyanate in the non-polar solvent is from 5 to 15 wt. % at room temperature and the dielectric constant of the non-polar solvent is below 2.10. 
     
     
       9. The process of claim 8 wherein the dielectric constant of the moderately polar solvent is from 2.24 to 10. 
     
     
       10. The process of claim 1 wherein the dielectric constant of the moderately polar solvent is from 2.24 to 10. 
     
     
       11. The process of claim 1 wherein the moderately polar solvent is a ketone having a dielectric constant up to 22. 
     
     
       12. The process of claim 1 wherein the non-polar solvent is selected from the group consisting of petroleum ether, ligroin, petrol and mixtures thereof. 
     
     
       13. The process of claim 12 wherein the moderately polar solvent is toluene. 
     
     
       14. The process of claim 1 wherein the moderately polar solvent is toluene. 
     
     
       15. The process of claim 1 wherein the temperature is maintained below 30° C. during the dimerization reaction and any subsequent purification or separation. 
     
     
       16. The process of claim 1 wherein the catalyst is a trialkyl phosphine. 
     
     
       17. The process of claim 16 wherein the catalyst is employed in an amount which is from 0.1 to 1.0 wt. %. 
     
     
       18. The process of claim 1 wherein from 1 to 2 parts by weight of the non-polar solvent is stirred with one part of 4,4'-diphenylmethane-diisocyanate at room temperature up to the saturation point and then moderately polar solvent is added until a homogeneous solution is produced. 
     
     
       19. The process of claim 1 wherein the dimerization catalyst is tributyl phosphine. 
     
     
       20. The process of claim 1 wherein the product 4,4'-diphenylmethane-uretdione-diisocyanate is separated from any unreacted material or by-products at a temperature less than 50° C.